1. Field of the Invention
This invention relates to an apparatus for forming electrophotographic images.
2. Description of the Prior Art
In general, in an electrophotographic image forming apparatus such as a color copier or color printer, images of at least two different colors formed on an electrophotographically-sensitive medium are transferred to a transfer medium in a multi-transfer manner so that a color image can be obtained on the transfer medium.
Transfer methods used for this process are known, such as one in which a transfer medium supported on an electrically conductive roller is transported in synchronism with a sensitive medium while being pressed thereagainst and a bias voltage of a polarity reverse to that of the toner is applied to the roller so as transfer the toner image to the transfer medium, or one in which a transfer medium is transported in synchronism with a sensitive medium while being supported on a cylindrical drum and a bias voltage of a polarity reverse to that of the toner is applied to the reverse surface of the transfer medium roller so as to transfer the toner image to the transfer medium.
In either case, it is desired that the bias roller or cylindrical drum provided as a support for the transfer medium is rotated in complete synchronism with the speed at which the surface of the sensitive medium moves.
This is because failure to attain rotational synchronism between the transfer medium support and the sensitive medium causes distortions and fading on the image at the time of each toner image transfer, this finally appearing as errors in color distribution in the final image formed on the transfer medium. In the case of an ordinary monochrome copier, a defect of this kind that is comparatively low significance is not easy to perceive because of the monochromatic tone of the image. However, in the case of a color copier or color printer, a defect of the same sort is highly noticeable because it is perceived as a difference in color. In this case, it is necessary to guarantee the rotational synchronism between the transfer medium support and the sensitive medium.
On the other hand, to hold the transfer medium on the bias roller or the cylindrical drum, a transfer-medium gripping means (e.g., a gripper opened or closed by a cam) is used.
Conventionally, there is a problem arising from the impact that occurs when the gripper is opened or closed by the cam. This impact causes abrupt changes in the load imposed on the rotating means and, hence, changes in the speed of rotation of the rotary drum, thereby damaging the rotational synchronism of the rotary drum and the sensitive drum. This problem can be relieved to some extent by optimizing the shape of the cam, but the effect of such efforts is limited. In another possible approach, an inertial body which functions as a fly-wheel rotating integrally with the transfer drum without any rattling may be attached to the transfer drum.
However, the provision of a fly-wheel on the transfer drum in this manner involves the following problems.
There is a need for the provision of a mechanism which enables the transfer drum to be easily detached from the main body of the apparatus in order to remove the transfer medium when the apparatus detects an abnormality and stops while the transfer medium is held on the transfer drum (jam disposal), or in order to satisfy certain requirements for maintenance. The provision of a fly-wheel entails an increase in the weight of the transfer drum and an increase in the are occupied (volume), resulting in a considerable deterioration in operability.
There are types of exposure system that may be applied to image forming apparatus represented by the above-described color copier or color printer. Examples of such systems are a slit exposure system in which an original-scanning optical system and the sensitive drum are driven in synchronism with each other by using a mechanical link means such as a gear train, chain, or timing belt and the sensitive medium is exposed to the original image through lenses, and a line-scanning exposure system in which the sensitive medium is scanned with information light at a constant speed by an exposure means which uses a laser scanner and a liquid crystal shutter and which is disposed above the sensitive drum.
In either case, there are problems, which will be described below with reference to FIG. 4.
In an electrophotography apparatus shown in FIG. 4, a development sleeve 13a for supplying a developer to the sensitive drum 1 and a spacer roller 13b provided as a means for constantly maintaining a gap .DELTA.l are in contact with the sensitive drum 1. Vibrations caused by a development device 13 are thereby transmitted to the sensitive drum, and the sensitive drum is vibrated (in the arrowed direction A in FIG. 4).
A cleaning blade 271 for removing untransferred toner is also in contact with the sensitive drum 1. This contact tends to cause vibrations of the blade, depending on the amount of toner remaining. These vibrations result in vibrations of the sensitive drum, as in the case of the development device.
Such vibrations of the sensitive drum act to cause non-uniformity of an image formed on the drum. In particular, in the case of the above-mentioned line-scanning exposure apparatus, vibration of the sensitive drum causes variations of the line interval (unevenness of the pitch), and changes in the image quality due to such exposure are more noticeable than those in a slit exposure apparatus. In the case of a line-scanning color printer, a deterioration in the image quality due to the above-described factors is particularly detrimental.
Moreover, vibration of the sensitive drum affects the transfer of a toner image from the sensitive drum to the transfer medium so as to cause errors in transfer matching, which, together with the above-mentioned unevenness of exposure, considerably reduces the quality of the image obtained.
On the other hand, vibrations which occur on the transfer drum as described above are also transmitted to the sensitive drum, thereby causing non-uniformity of the image at the time of the exposure which is performed in the above-described manner.